This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-279209, filed Sep. 30, 1999; No. 11-309062, filed Oct. 29, 1999; No. 2000-214589, filed Jul. 14, 2000; No. 2000-214590, filed Jul. 14, 2000; No. 2000-224316, filed Jul. 25, 2000; and No. 2000-234378, filed Aug. 2, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a liquid crystal display apparatus performing a reflection display by utilizing an external light.
A liquid crystal display apparatus comprises a liquid crystal element and a pair of polarizing plates arranged to have the liquid crystal element sandwiched therebetween. The liquid crystal element comprises a front transparent substrate on the side of the observation of the display, a rear transparent substrate positioned to face the front transparent substrate, two electrodes mounted to the inner surfaces of the front and rear transparent substrates, respectively, and a liquid crystal layer interposed between these two electrodes. The polarizing state of the transmitting light is controlled by the liquid crystal layer in accordance with the electric field applied between the two electrodes.
A TN (twisted nematic) type is widely used as such a liquid crystal display apparatus. The TN type liquid crystal display apparatus is constructed such that the liquid crystal molecules of the liquid crystal layer included in the liquid crystal element are aligned under the state free from an electric field in a twisted manner with a twisting angle of about 90xc2x0. Also, the paired polarizing plates are arranged such that the transmitting axes thereof are substantially parallel to each other or substantially perpendicular to each other.
The liquid crystal display apparatus includes a transmission type in which a transmission display is performed by utilizing an illuminating light emitted from a back light and a reflection type in which a reflection display is performed by utilizing the external light in the environment of use of the liquid crystal display apparatus. The transmission type liquid crystal display apparatus consumes a large amount of an electric power for lighting the back light and, thus, the reflection type liquid crystal display apparatus performing the reflection display by utilizing an external light is advantageous in terms of the power saving.
The conventional reflection type liquid crystal display apparatus is constructed such that a reflecting plate is arranged behind the rear polarizing plate that is arranged behind the liquid crystal element.
In the conventional reflection type liquid crystal display apparatus, the external light incident on the front transparent substrate on the side of observation of the display is allowed to be incident as a linear polarized light on the liquid crystal element by the polarizing function of the front polarizing plate arranged in front of the liquid crystal element. The polarized state of the light incident on the liquid crystal element is controlled by the liquid crystal layer and, then, emitted to the rear side of the liquid crystal element. Among the light emitted to the rear side of the liquid crystal element, the light component transmitted through the rear polarizing plate is reflected by the reflecting plate, and the reflected light is allowed to be transmitted through the rear polarizing plate, the liquid crystal element and the front polarizing plate so as to be emitted to the front side of the display apparatus.
In the conventional reflection type liquid crystal display apparatus, the light of the polarized component along the transmission axis of the front polarizing plate, which is contained in the light incident from the front side, is transmitted through the front polarizing plate so as to be incident on the liquid crystal element, and the light of the polarized component along the absorption axis of the front polarizing plate is absorbed by the front polarizing plate. It follows that about half the light incident from the front side is absorbed by the front polarizing plate and, thus, is made useless. As a result, the conventional reflection type liquid crystal display apparatus is poor in its utilization rate of the incident light, and the light reflected by the reflecting plate so as to be emitted to the front side has a high intensity, resulting in failure to obtain a bright screen.
An object of the present invention is to provide a liquid crystal display apparatus that permits increasing the brightness of the screen by effectively utilizing the external light incident from the front side and also permits obtaining a good contrast.
According to an aspect of the present invention, which has been accomplished in an attempt to achieve the above-noted object, there is provided a liquid crystal display apparatus, comprising:
a liquid crystal element including a front substrate positioned on the side of an observer and having a first electrode mounted to one surface, a rear substrate having a second electrode arranged to face the first electrode, and a liquid crystal layer interposed between these substrates, the liquid crystal layer controlling the polarized state of the transmitted light in accordance with the electric field applied between the first and the second electrodes;
a first reflection polarizing plate arranged on the front side of the liquid crystal element and reflecting the light of one of the two polarized components of the incident light, the two polarized components being perpendicular to each other, and transmitting the light of the other polarized component; and
a rear member arranged behind the liquid crystal element and reflecting at least a part of the light transmitted through the liquid crystal element so as to be emitted to the rear side.
The liquid crystal display apparatus further comprises a diffusion means arranged on the front side of the reflection polarizing plate and a diffusion layer arranged between the reflection polarizing plate and the liquid crystal element and/or between the liquid crystal element and the rear member. It is desirable for the diffusion layer to be formed of a lens film having micro lenses arranged on one surface.
In the liquid crystal display apparatus of the present invention, another reflection polarizing plate, which transmits the light of one of the two polarized components, which are perpendicular to each other, of the incident light and reflects the light of the other polarized component, is used as the rear member. When the liquid crystal element is of a TN type in which the liquid crystal molecules are twist-aligned with a twisting angle of 90xc2x0, the first reflection polarizing plate arranged on the front side of the liquid crystal element is arranged such that the transmission axis of the first reflection polarizing plate is substantially parallel to perpendicular to the aligning direction of the liquid crystal molecules in the vicinity of the front substrate of the liquid crystal element. On the other hand, the second reflection polarizing plate constituting the rear member is arranged such that the transmission axis of the second reflection polarizing plate is substantially parallel or perpendicular to the transmission axis of the first reflection polarizing plate. Further, it is possible to arrange a back light on the front side of the rear member.
The rear member is constructed by a reflection polarizing plate and a light absorbing means. The light absorbing means is formed of a light absorbing film or an absorption polarizing plate that absorbs the light of one the two polarized components, which are perpendicular to each other, of the incident light and transmits the light of the other polarized component. Also, the rear member is constructed by the absorption polarizing plate and a reflection plate arranged on the rear side of the absorption polarizing plate.
According to the liquid crystal display apparatus of present invention, the reflection polarizing plate that does not absorb the light and has a high transmittance is arranged on the front side, i.e., the observing side, of the liquid crystal element, making it possible to increase the intensity of the light incident on the liquid crystal element from the front side. Naturally, the light returned to the observing side has a high intensity so as to obtain a bright display. It should also be noted that the light of one polarized component of the incident light from the front side is reflected by the reflection polarizing plate so as to be emitted to the front side. Since the reflected light increases the brightness of the entire screen, the incident light from the front side can be effectively utilized so as to obtain a bright screen.
It should also be noted that, in the liquid crystal display apparatus of the present invention, an optical element, which transmits the light incident from the front side so as to be incident on the reflection polarizing plate and which permits the light of one polarized component reflected from the reflection polarizing plate to be incident again on the reflection polarizing plate with the polarized state changed, is arranged on the front side of the reflection polarizing plate arranged on the observing side of the liquid crystal element. As a result, the external light can be utilized more efficiently so as to achieve a bright display. In this case, the optical element is constructed by a transparent film or a retardation plate. Preferably, it is desirable for the optical element to be formed of a quarter wavelength plate having a phase difference of 1/4 wavelength, which is arranged such that the retarded phase axis of the quarter wavelength plate crosses both the reflection axis and the transmission axis of the reflection polarizing plate arranged on the front side of the liquid crystal element with a crossing angle of about 45xc2x0.
When it comes to the liquid crystal display apparatus having the optical element arranged on the front side of the reflection polarizing plate, it is desirable to arrange a means, which allows the light of one polarized component reflected from the reflection polarizing plate to be incident on the optical element arranged on the front side of the reflection polarizing plate at an angle of incidence at which the incident light is subjected to the inner surface reflection by the optical element, between the reflection polarizing plate and the optical means. It is possible for the particular means to be formed by applying a surface treatment to the reflection polarizing plate. Alternatively, a diffusion layer arranged between the reflection polarizing plate and the optical means can be used as the particular means. The particular means has a directivity in a direction inclined from a line normal to the reflection polarizing plate arranged on the front side of the liquid crystal means.
It is desirable to apply the surface treatment to the reflection polarizing plate in a manner to form a fine irregularity that diffuses and reflects the light of one polarized component and transmits the light of the other polarized component without diffusing the light.
A diffusion means is arranged on the front side or rear side of the liquid crystal element of the liquid crystal display apparatus. The diffusion means arranged on the rear side of the liquid crystal element has a directivity in a direction parallel to a line normal to the reflection polarizing plate. The directivity can be obtained by a lens film having micro lenses arranged on surface.
The rear member of the liquid crystal display apparatus is constructed by a reflection polarizing plate or a means for reflecting the light of one of the two polarized components, which are perpendicular to each other, of the incident light and for absorbing the light of the other polarized component. The particular means is constructed by a reflection polarizing plate and a light absorption means. It is possible for the light absorption means to be formed of a light absorbing layer, an absorption polarizing plate, or a colored film absorbing light having a predetermined wavelength. Also, the rear member is constructed by a reflection film, a combination of an absorption polarizing plate and a reflection film, or two reflection polarizing plates arranged with a diffusion layer interposed therebetween and having a light absorption means arranged on the rear side of the reflection polarizing plate. It is desirable for the rear member to exhibit diffusion-reflection characteristics.
In the liquid crystal display apparatus of the present invention, it is desirable for the liquid crystal element to be formed such that, in the case of a simple matrix type, the liquid crystal molecules are twist-aligned with a twisting angle of about 1000xc2x0, and the product xcex94nd between the refractive index anisotropy xcex94n of the liquid crystal layer and the thickness d of the liquid crystal layer falls within a range of between 115 nm and 130 nm.
According to the liquid crystal display apparatus of the present invention, a reflection polarizing plate is used as a polarizing element arranged on the front side of the liquid crystal element, and the optical element is arranged on the front side of the reflection polarizing plate. The particular construction makes it possible to allow both the light of one of the two polarized components, which are perpendicular to each other, of the external light incident from the front side, the one polarized component being transmitted through the reflection polarizing plate, and the light of the polarized component of the light reflected from the reflection polarizing plate and also reflected from the inner surface of the optical element, the polarized component being transmitted through the reflection polarizing plate, to be transmitted through the reflection polarizing plate so as to be incident on the liquid crystal element. It follows that the external light incident from the front side can be utilized with a high efficiency so as to achieve a bright display of a high brightness.
In addition, since a retardation plate is used as the optical element, the polarized state of the light of one polarized component reflected from the reflection polarizing plate can be changed into the light of a polarized state that can be transmitted through the reflection polarizing plate so as to permit the light to be incident again on the reflection polarizing plate. It follows that the amount of the light incident on the liquid crystal element is increased. It is also possible to decrease the light leakage that the light reflected from the reflection polarizing plate is transmitted through the optical means so as to be emitted to the front side. It follows that the amount of the light transmitted through the liquid crystal element so as to be emitted to the observing side is increased so as to suppress the xe2x80x9cfloatingxe2x80x9d of darkness of the dark display caused by the light leakage. As a result, it is possible to obtain a bright screen having a good contrast.
Further, in the liquid crystal display apparatus of the present invention, it is possible to use as the liquid crystal element a so-called xe2x80x9cSTN typexe2x80x9d liquid crystal element having a liquid crystal layer in which light crystal molecules are twist-aligned between a pair of substrates with a twisting angle of 180xc2x0 to 270xc2x0.
In this case, the incident light can be effectively utilized by arranging a transparent film that permits the light reflected from the reflection polarizing plate to be subjected to the inner surface reflection so as to be incident again on the reflection polarizing plate. It is desirable for the transparent film to exhibit optical characteristics of changing the polarized state of the transmitted light. The particular optical characteristics can be achieved by using a quarter wavelength plate. The quarter wavelength plate should be arranged such that the retarded phase axis thereof crosses the transmission axis of the reflection polarizing plate arranged on the front side of the liquid crystal element with a crossing angle of about 45xc2x0. It is possible to further improve the efficiency of utilizing the external light by arranging a diffusion means between the transparent film and the reflection polarizing plate.
Also, in the liquid crystal display apparatus of the present invention, an absorption polarizing plate and/or a diffusion layer, which has a transmission axis transmitting one of the two polarized components, which are perpendicular to each other, of the incident light and an absorption axis absorbing the light of the other polarized component, is arranged on at least one of the front side (or observing side) and the rear side of the STN type liquid crystal element. Further, a reflection polarizing plate or a reflection film is arranged on the rear side of the STN type liquid crystal element.
According to the liquid crystal display apparatus of the present invention, the light of one of the two polarized components, which are perpendicular to each other, of the incident light, the polarized component being transmitted through the reflection polarizing plate, is transmitted through the reflection polarizing plate so as to be incident on the STN type liquid crystal element. Also, the light reflected from the reflection polarizing plate is subjected to the inner surface reflection by the transparent film so as to be incident again on the reflection polarizing plate. Further, the light of the polarized component transmitted through the reflection polarizing plate is transmitted through the reflection polarizing plate so as to be incident on the liquid crystal element. It follows that a large amount of light is incident on the liquid crystal element, making it possible to utilize effectively the incident light from the front side and, thus, to obtain a bright screen.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.